This invention relates to a method for microencapsulating water-soluble or water-dispersible or water-sensitive materials in an organic continuous phase by urethane coacervation. The method is useful for the microencapsulation of a variety of materials, e.g., fluorescent and phosphorescent pigments, and is particularly useful for the microencapsulation of breaker materials used to break fracturing fluids employed in the stimulation of subterranean formations.
The use of capsules to mask, protect, stabilize, delay and control the release of various materials is well known and, in particular, the use of such capsules or microcapsules to encapsulate breaker materials has been described in, e.g., U.S. Pat. Nos. 4,741,401 to Walker et al; 4,919,209 to King; 5,110,486 to Manalastar et al; 5,102,558; 5,102,559; 5,204,183 and 5,370,184 all to McDougall et al; 5,164,099 and 5,437,331 to Gupta et al; and 5,373,901 to Norman et al.
Typically, the encapsulated breaker material is formed by surrounding the breaker material with an enclosure member which is sufficiently permeable to at least one fluid, generally water, found in a subterranean formation being treated or to a fluid injected with the capsule into the formation and which is capable of releasing the breaker. Generally the breaker is coated or encapsulated by spraying small particles of the material with a suitable coating formulation in a fluidized bed or by suspension polymerization wherein the breaker particles are suspended in a liquid-liquid system containing a monomer which is capable of polymerizing to form a polymeric coating surrounding the breaker particle.
Although the foregoing methods appear to provide satisfactory encapsulated materials, it remains desirable to provide an alternative method which is more economical and which gives equivalent or superior performance.